In an atomic force microscope, small scale variations are detected in the position of a probe tip that is in close proximity to a sample being scanned. Depending on the nature of the interaction, the tip is operated either in a dynamic or static mode. Consequently, it is desirable to sense the motion of the tip with a system which is capable of detecting AC, DC and quasi-DC (slowly varying) changes in position of the tip. A fundamental consideration for such sensing systems is the need for detection sensitivities in the range of minute fractions of a nanometer. This clearly implies an immunity to environmental changes such as temperature and pressure induced microphonics.
A variety of techniques have been employed to enable high precision position sensing, e.g., AC laser heterodyne interferometers; reflections of a focused laser beam off a surface; an all-fiber optic Sagnac interferometer, etc. Heterodyne interferometers are complicated systems requiring precise alignment of their components. Laser reflection techniques are limited in their capacity to reject microphonics. Fiber optic systems are potentially more compact, however, neither they nor heterodyne interferometers respond to DC or quasi-DC deflections.
One exception is a fiber system in which interference occurs between a sample's reflected light and that reflected off a fiber end. However, in such a system there is no direct way of insuring continuous operations at a maximum, stable, sensitivity. (see Rugar et al, "Force Microscope Using a Fiber-Optic Displacement Sensor", Review of Scientific Instruments 59(11) November, 1988, pp. 2337-2340.
The patent art is replete with interferometric systems which have been applied to various applications. In U.S. Pat. No. 4,717,255 to Ulbers, an optical interferometer is shown that measures the position of a sensing tip that is following an irregular surface. It does not consider any method for the elimination of residual drift due to microphonics. U.S. Pat. No. 4,671,659 to Rempt et al. describes an optical interferometer that uses expansions and contractions of a test article to cause changes in the length of a measurement fiber that is adhered thereto. U.S. Pat. No. 4,652,129 to Martinelli, describes a Michelson interferometer that combines a reflected measurement signal with a reference signal and thereby enables induced noise to be suppressed. U.S. Pat. No. 4,753,529 to Layton describes a Mach-Zehnder interferometer wherein a difference in pathlength between a pair of fibers is determined by alteration of the optical source frequency.
U.S. Pat. No. 4,378,497 to Giallorenzi employs a magnetostrictive element to stretch a fiber section to enable a change in phase shift therethrough. No distinction is made between laser fluctuations and low frequency fluctuations in the feedback system that is used to control the magnetostrictive element. U.S. Pat. No. 4,799,797 to Huggins describes a multiplexed optical sensor system employing an interferometric arrangement. It employs phase error compensation that is based on a DC technique which is susceptible to laser light fluctuations. U.S. Pat. No. 4,603,296 to Koo et al. is a further interferometer that employs DC compensation. This interferometer is used as a magnetometer.
U.S. Pat. No. 5,017,010 to Mamin et al describes an interferometer that is applied to the sensing of an atomic force microscope probe. The system employs light reflected from the probe (and from the polished end of a fiber adjacent the probe) to achieve an interferometric combination of reflected beams. A photodetector senses a portion of the injected light as a reference which is used subtractively to negate power fluctuations of the laser. European published patent application WO 83/03684 to Kino et al illustrates a Sagnac interferometer that is employed to determine surface changes occurring in a surface acoustic wave device. Because the system is a Sagnac interferometer, it is not able to make DC or low frequency measurements.
The following prior art describes interferometers which employ correction systems for thermal and microphonic effects. U.S. Pat. No. 4,627,728 to Willson employs two Fabry-Perot interferometers, one for sensing and the other for correction purposes. The system is employed for magnetic field strength sensing. U.S. Pat. No. 4,471,219 to Giallorenzi is a magnetic field sensor which includes an acoustic circuit for nulling responses in the detector due to acoustic perturbations. One embodiment modulates the axial alignment of fibers in an end to end configuration by means of magnetic material. A second embodiment modulates the coupling between the fibers using a magnetostrictive material. U.S. Pat. No. 4,759,627 to Thylen et al describes a Mach-Zehnder interferometer that employs a difference signal for compensation. The compensation signal is a representation of the parameter to be measured and no distinction is made between spurious variations and the wanted signal.
U.S. Pat. No. 4,486,657 to Bush describes a fiber optic acoustic sensing system wherein one modulator maintains an interferometer locked in-phase by effectively cancelling out a phase change produced by temperature and acoustic pressure fluctuations. To effect this cancellation, the modulator inversely duplicates the phase shift to produce the desired output signal. U.S. Pat. No. 4,530,603 of Shaw et al. describes a fiber optic sensor that includes a feedback system for stabilizing a fiber loop against low frequency thermal drift. The system is employed to sense acoustic waves that modify the phase shift through the fiber loop.
Other prior art describing interferometer systems with sensitivity and stabilization adjustments means can be found in the following U.S. Pat. Nos.: 4,918,371 to Bobb; 4,887,901 to Falco et al; 4,868,381 to Davis; 4,853,534 to Dakin; and 4,885,462 to Dakin.
As above indicated, a substantial body of prior art exists evidencing interferometric systems that are useable for position sensing; surface sensing; employ feedback to control microphonics and thermal-induced fiber variations; and also employ piezoelectric feedback compensation. However, a need still exists for an optical interferometer that is capable of detecting both AC and DC dimensional variations; exhibits low drift; and is inexpensive and of simple design.
Accordingly, it is an object of this invention to provide an optical interferometer that exhibits low drift and is particularly adapted to position displacement measurements.
It is another object of this invention to provide an optical interferometer that is able to reliably detect both AC, DC and quasi-DC dimensional changes.